Directional drilling, such as for the recovery of hydrocarbons or minerals from a subsurface formation, is typically carried out using a downhole motor (also commonly referred to as a “drilling motor” or “mud motor”), which is incorporated into the drill string above the drill bit. A downhole motor may include several primary components (in order, starting from the top of the downhole motor assembly): a top sub adapted to facilitate connection to the lower end of a drill string (“sub” being the common general term in the oil and gas industry for any small or secondary drill string component); a power section; a drive shaft enclosed within a drive shaft housing, with the upper end of the drive shaft being operably connected to the rotor of the power section; and a bearing assembly, which may include a mandrel with an upper end coupled to the lower end of the drive shaft, plus a lower end adapted to receive a drill bit or other components attached to a drill bit.
During operation of the downhole motor, high-pressure drilling fluid is forced through the power section, causing the rotor to rotate within the stator. As the drill bit engages the formation for drilling, torque is then required to turn the bit against the formation. This reactive torque induces a pressure drop across the power section (i.e., the drilling fluid pressure being lower at the bottom, or output end, of the power section than at the top, or input end, of the power section). The power thus delivered to the rotor output shaft is proportional to the product of the volume of fluid passing through the power section multiplied by the pressure drop across the power section (i.e., from fluid inlet to fluid outlet). Further, the power delivered to the rotor output shaft is proportional to the product of the rotational speed of the rotor and the torque required to rotate the drill bit. Accordingly, a higher rate of fluid circulation fluid through the power section will result in a higher rotational speed of the rotor within the stator, and correspondingly higher power output. Likewise, for a given rotor speed, higher torque output also results in a correspondingly higher output power.
The output shaft of the power section rotor is coupled to the upper end of the drive shaft, for transmission of rotational torque to turn the drill bit. However, the motion of the rotor in a positive displacement-type downhole motor is eccentric in nature, or “precessional.” In operation, the longitudinal axis of the rotor precesses, or orbits, about the longitudinal axis of the stator housing with rotor axis and the stator axis remaining parallel with each other. At the same time, the rotor also rotates about its own longitudinal axis. This description applies to a positive displacement motor commonly referred to as a “Moineau” motor; however, the term “downhole motor” is not limited to positive displacement motors and may include, for example, turbodrills, in which the rotor motion is concentric.
The output shaft of the rotor is operationally coupled to the upper end of the drive shaft by way of a first (or upper) universal joint, whereby rotation and torque can be transferred from the rotor to the drive shaft irrespective of the fact that the rotor and drive shaft axes may be non-collinear.
In recent years, power sections have been introduced that generate very high-torque. These include “even-wall” stators such as the ERT series offered by Robbins & Myers, and hard rubber (HR) stators such as those offered by Dyna-Drill. Higher torque results from the ability of these power sections to withstand higher operating pressures and pressure drops. Necessarily, the operating pressure of these power sections also produces high axial thrust. Typical prior art universal joints, an example of which is found in U.S. Pat. No. 5,267,905, use ball bearings to transmit both torque and thrust. The bearing(s) used in the universal joints as drive elements to transmit torque must endure high loads and a fretting motion, which create point contact and high Hertzian stresses that may cause the mating materials to yield or spall. Also, when used as thrust bearings, ball bearings and their mating thrust seats may suffer galling because the thrust balls must be relatively small, because they are positioned under, and in the same plane with, the drive elements. Spalling and galling are destructive occurrences that can lead to costly failure of the universal joint, and thus, of the entire mud motor.